Many radio frequency (RF) communications protocols rely upon amplitude modulation (AM) of an RF carrier signal to encode data therein. As RF communications protocols evolve, the required accuracy of this amplitude modulation continues to increase. For RF power amplifiers (PA), the linearity of the RF PA determines its ability to accurately replicate the amplitude of an RF input signal in proportion to the gain of the RF PA. Accordingly, the linearity of an RF PA is determined by the range of RF input signal power over which the gain response of the RF PA remains constant. Generally, the gain response of an RF PA is relatively flat over a range of RF input signal amplitudes, then tapers off at a certain point. One way to increase the range of input signals over which an RF PA produces a flat gain response is to increase a power supply voltage provided thereto. Known in the industry as increasing a voltage “headroom” of an RF PA, increasing the power supply voltage provided to an RF PA effectively extends the range of RF input signal power over which the gain response of the RF PA remains constant. Due to physical limitations of the RF PA, however, increasing the power supply voltage provided to the RF PA will only extend the linearity of the RF PA so far. Further, as the gain of the RF PA is increased, the range of RF input signal power over which the gain response of the RF PA remains constant decreases.
FIG. 1 shows a schematic of conventional RF PA circuitry 10. The conventional RF PA circuitry 10 includes a driver amplifier stage 12, an output amplifier stage 14, a power supply voltage VDD, and a choke inductor L_CH. FIGS. 2 and 3 show various operating conditions of the conventional RF PA circuitry 10. In operation, an RF input signal RF_IN is received at an input of the driver amplifier stage 12, where it is amplified and delivered to the output amplifier stage 14. The output amplifier stage 14 further amplifies the RF signal to provide an amplified RF output signal RF_OUT. The power supply voltage VDD provides power for amplification. As shown in FIG. 2, as the power supply voltage increases from V1 to V3, the range of input signal amplitudes over which the gain of the conventional RF PA circuitry 10 remains constant increases as discussed above. However, increasing the power supply voltage beyond a certain point, for example, V3, no longer has an effect on the linearity of the conventional RF PA circuitry 10 as discussed above. Further, as shown in FIG. 3, as the gain of the conventional RF PA circuitry 10 increases, the range of RF input signal power over which the gain of the conventional RF PA circuitry 10 remains constant decreases. Accordingly, in situations in which a high power input signal and/or high gain of the conventional RF PA circuitry 10 is required, the linearity of the conventional RF PA circuitry 10 will generally suffer.
In light of the above, there is a present need for RF PA circuitry with further improvements to the linearity thereof.